Abusing the C preprocessor

August 29, 2011

Both tricks shown here are related with a change (by Peter Zijlstra) in the kmap_atomic() and kunmap_atomic() macros/functions. LWN has an excellent article about what that change involved. It basically ‘dropped’ support for atomic kmap slots, switching to a more general stack-based approach.

Now with this change, the number of arguments passed to the kmap_atomic() function changed too, and thus you end up with a huge patch covering all the tree, which fixed the issue (changing kmap_atomic(p, KM_TYPE) to kmap_atomic(p)).

But there was another way to go. Some C preprocessor magic.

#define kmap_atomic(page, args...) __kmap_atomic(page)

Yes, the C preprocessor supports va_args. :)
(which I found out when going through the reptyr code, but I’ll talk about it in an other post.)

Today, I saw a thread at the lkml, which actually did the cleanup I described. Andrew Morton responded:

I’m OK with cleaning all these up, but I suggest that we leave the back-compatibility macros in place for a while, to make sure that various stragglers get converted. Extra marks will be awarded for working out how to make unconverted code generate a compile warning

And Nick Bowler responded with a very clever way to do this (which involved abusing heavily the C preprocessor :P):

The actual warning is printed due to the deprecated attribute of the foo_unconverted() function.

The fun part, however, is how we get to use the foo ‘identifier’/name to call either foo() or foo_uncoverted() depending on the number of arguments given. :)

The trick is to use the __VA_ARGS__ to ‘shift’ the numbers 9-0 in the NARG macro, so that when calling the NARG_ macro, _9 will match with the first __VA_ARGS__ argument, _8 with the second etc, and so n will match with actual number of arguments (I’m not sure I described it very well, but if you try doing it by hand, you’ll understand how it’s working).

Now that we have the number of arguments given to foo, we use the PASTE macro to ‘concatenate’ the number of the arguments with the function name, and the actual arguments given, and call the appropriate wrapper macro (foo1, foo2 etc).

Another interesting thing, which I didn’t know, is about argument expansion in macros. For macros that concatenate (##) or stringify (#) the arguments are not expanded beforehand. That’s why we have to use PASTE2 as a wrapper, to get the NARG() argument/macro fully expanded before concatenating.

Ok, C code can get at times a bit obfuscated, and yes you don’t have type safety etc etc, but, man, you can be really creative with the C language (and the C preprocessor)!
And the Linux kernel development(/-ers) prove just that. :)